What are I/O registers?
addresses associated with I/O devices
What is a driver?
Software that communicates with an I/O device. Other programs call functions in the device driver to access the device without having to understand lower-level hardware
What is an embedded I/O system?
a programmed controller (comp./microcont.) with a dedicated function within a larger mechanical/electrical system, often with real-time computing constraints
What does an embedded I/O system usually include?
- input device (peripheral)
- controller (computer/microcontroller)
- output device (peripheral)
MCU - GPIO
- used to write/read digital signals
- requires registers to read input, pin values, write output in values and set directions of the pin
- requires configuration registers (interrupts, pin type, rising falling edge)
What is the purpose of an I/O interface?
To manage communication between peripherals and the computer. It resolves differences between the peripherals and CPU
What are the four major differences between peripherals and computers?
1) data format in peripherals, word format in CPU and memory
2) operating modes of peripherals differ, must be controlled
3) peripherals are often electromechanical devices which operate differently from CPU and memory (electronic devices), therefore conversion signals may be required
4) data-transfer rate of peripherals and clock rate of CPU, synchronisation required
What are interface units?
- special hardware components between the CPU and peripherals which supervise/synchronise all input and output transfers
- interface between CPU bus and peripherals
What is the purpose of the clock?
- synchronises all data transfer
- generates signal for timers, PWM and other peripherals
Give two clock sources
- mechanical resonant devices such as crystals or ceramic resonators
- RC oscillators
What is the purpose of prescaling?
- used to divide the clock seed the timer as it is too high for counter register (16 bit) to hold
What is the Auto Reload Reigster (ARR)
- further divide system clock to give timer frequency
- counter counts up to value in ARR before reloading
pulse width
“on time”
duty cycle
pulse width divided by period
What is the Capture Compare Register (CRR)?
- compared to counter and raises a signal when equal
- one CRR for each channel
- used to change pulse with
What happens when an external interrupt is raised?
The MCU stops its current task, branches to an ISR which gets executed, then returns to previous task
What are the different standards for Serial I/O
- SPI
- UART (RS 232/485)
- I2C
Describe SPI
Interface requires three lines minimum:
- SCK
- MOSI
- MISO
- CE/CS which can be used to select a peripheral
Describe an SPI transmission:
- master (MCU) produces clock signal
- master initiates communication with series of clock pulses
- master puts data on MOSI starting with m.s.b (sending)
- slave puts data on MISO (receiving)
Describe UART
- communication without a clock, therefore devices aren’t synchronised (asynchronous)
- system agrees on data rate, each device generates its own clock
- slower than SPI
- protocols include RS-232 and RS-485
What is the purpose of the parity bit in RS-232 protocol for UART? How does it work?
- used to detect data corruption
- configured as even or odd
- for even parity, parity bit is chosen so total collection of data (incl. parity bit) has an even number of 1’s
- receiver checks if even number of 1’s was received and signals an error if not
- odd parity is reverse
What is Baudrate (RS-232 protocol)?
- signalling rate measured in symbols/sec
- multi-level signalling (multiple bits per symbol)
- 2-level signalling (each symbol represents a bit)
RS-485 protocol for UART
- does not define a protocol, defines an electrical interface
- uses differential signalling to enable data transmission over long distances
- noise rejection
Describe I2C
interface requires two lines, one for bidirectional data signals (SDA), one for the clock signal (SCK). Bus drivers are open drain, can pull signal line low but not drive it high, so each signal line requires a pull-up resistor
